Many organic compounds serving as photoconductive materials are known, and some of them have been confirmed to have considerably high photosensitivity. Under the present situation, however, it is rare to find a practical application of the organic photoconductive materials to electrophotographic materials.
Organic photoconductive materials have many excellent properties as compared with inorganic ones and are broadly applicable in the technical field of electrophotography. For example, it is possible to produce transparent electrophotographic photosensitive films, flexible electrophotographic photosensitive films or electrophotographic photosensitive films which are light-weight and easy to handle only when organic photoconductive materials are used. Further, organic photoconductive materials have many characteristic properties that can hardly be expected from the inorganic materials, such as film-forming properties required in the production of electrophotographic photosensitive materials, surface smoothness, selectivity of polarized charge when applied to an electrophotographic process, and the like.
In spite of the above-described superiority of the organic photoconductive materials in many respects, their low photosensitivities and brittleness of their coating films constitute major reasons for their poor contribution to, and limited use in, electrophotographic techniques.
Studies on organic photoconductive materials have been directed, since the beginning, to low molecular weight heterocyclic compounds, nitrogen-containing aromatic compounds, various high molecular weight aromatic compounds, and the like, and some compounds having considerably high sensitivity have been investigated. In recent years, the center of the study has been shifted to processes of sensitization in an attempt to further increase sensitivity. This is because even the organic photoconductive materials having higher sensitivity than other materials so far known do not have sensitivity high enough to be practically used as is without being subjected to sensitization. Therefore, it is always essential for practical use of the organic photoconductive material to chose the most effective sensitization method to be applied. It is no exaggeration to say that industrial values of the organic photoconductive materials depend on the degree of increase in sensitivity finally reached by the sensitization method applied.
The most commonly known methods for sensitization include addition of sensitizing dyes and addition of Lewis acids. These methods are applicable to almost all of the organic photoconductive materials. The mechanism of sensitization of the former method is based on addition of spectral absorption characteristics of the dye to the organic photoconductive material, while that of the latter method is based on manifestation of new spectral sensitivity due to the formation of a donor acceptor complex between the organic photoconductive material and the Lewis acid.
On the other hand, electrophotographic systems using semi-conductor laser beams as a light source have recently been developed widely. Semi-conductor lasers have great advantages in that they are smaller-sized and can be produced at lower cost than gas lasers, e.g., an He-Ne laser, etc. and also they can be directly modulated. However, since many of the semi-conductor lasers have their oscillation wavelengths at 750 nm or more, photosensitive materials to be used in such systems should have their main absorption at 750 nm or more and exhibit high sensitivities. To this effect, researches have been conducted on various sensitizing dyes and others that would meet these requirements. However, most organic compounds exhibiting absorption in such a longer wavelength region are generally unstable to heat or humidity, which results in problems on production and handling. Moreover, photosensitive materials prepared by using such compounds often undergo serious changes with time, ultimately losing their functions as photosensitive materials when preserved for a long period of time. Further, some compounds, though exhibiting their main absorption in such a longer wavelength region, have small extinction coefficients or low electrophotographic sensitivities. Accordingly, the present state-of-the-art is not fully satisfactory, and it has been keenly desired to develop electrophotographic photosensitive materials showing higher sensitivity and higher durability.